The present invention is directed to switching-mode power conversion systems. More particularly, the invention provides systems and methods for adaptive switching frequency control. Merely by way of example, the invention has been applied to off-line switching-mode flyback power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
To meet certain international power conservation requirements, power supplies need to have very low levels of standby power consumption. For example, the averaged power efficiency should be high at 25%, 50%, 75%, and 100% load conditions. In switching-mode power conversion systems, power loss mainly results from the switching loss of the power switches, the conduction loss of the power switches, the core loss of the transformer and inductors, and the power loss of the snubbers. All of these types of power losses are proportional to the switching frequency. Additionally, the core loss of the transformer and inductors and the power loss of the snubbers also depend on the material used.
FIG. 1 is a simplified diagram showing a conventional flyback switching-mode power conversion system with fixed switching frequency. The flyback switching-mode power conversion system 100 includes an oscillator 110, a current generator 120, and a pulse-width-modulation (PWM) generator 130. The switching frequency of the PWM generator 130 is determined by the oscillator 110. For example, the oscillator 110 receives a predetermined constant current from the current generator 120. Hence the switching frequency is constant regardless of the input AC line voltage (i.e., VAC). The input AC line voltage is also called the line voltage.
FIG. 2 is a simplified diagram showing the conventional oscillator 110 in the conventional power conversion system 100. The oscillator 110 includes a charge current source IC and a discharge current source ID, which alternatively change the voltage of CO through switches SH and SL. Additionally, the oscillator 110 includes a RS flip-flop formed by NAND1 and NAND2 for latching switching states. Moreover, the oscillator 110 includes two voltage clamping comparators A1 and A2 with threshold voltages VH and VL respectively.
As shown in FIG. 2, the oscillator 110 outputs a ramping signal and a clock signal to the PWM generator 130. The oscillating amplitude of the ramping signal and the clock signal is equal to the difference between VH and VL. Also, the switching frequency fSW of the PWM generator 130 is determined as follows:
                              f          Sw                =                  1                                    T              ON                        +                          T              OFF                                                          (        1        )            
where
                              T          ON                =                                            (                                                V                  H                                -                                  V                  L                                            )                        ×                          C              O                                            I            C                                              (        2        )                                          T          OFF                =                                            (                                                V                  H                                -                                  V                  L                                            )                        ×                          C              O                                            I            D                                              (        3        )            
Hence, the switching frequency fSW depends on IC and OD.
Referring to FIG. 1, for low power applications, the power loss due to switching loss and the conduction loss due to one or more power switches depend on the line voltage. For example, in a given rated power converter, the switching loss dominates for high line voltages while the conduction loss dominates for low line voltages.
Specifically, the switching loss PSW often is given byPsw=½fSW·(VIN)2·(CDS+CDG)°½fSW·VIN·IPK(tSW—ON+tSW—OFF)  (4)
where fsw is the switching frequency, and VIN is an input voltage for the primary winding. As shown in FIG. 1, the input voltage VIN is the rectified line voltage. Additionally, CDS and CDG are the parasitic capacitance associated with the power switch. Moreover, IPK is the peak current for the primary winding at the switching transient, and tSW—ON and tSW—OFF are the switching transient times.
According to Equation 4, for a given power switch such as MOSFET, the switching loss increases with the input voltage as a quadratic function. Hence, lowering the switching frequency can reduce the switching loss, especially for high input voltages.
In another example, the conduction loss Pcond is
                              P          cond                =                                            1                              T                SW                                      ⁢                                          ∫                0                                  t                  on                                            ⁢                                                                    i                    L                    2                                    ⁡                                      (                    t                    )                                                  ⁢                                                                  ⁢                                                      ⅆ                    t                                    ·                  R                                                              =                                    (                                                I                  PK                  2                                -                                                      I                    PK                    2                                    ⁡                                      (                    0                    )                                                              )                        ·            R            ·                          t              on                        ·                          f              SW                                                          (        5        )            
where IPK is the peak current that the switch conducts at the end of the switch-on period, and IPK(0) is the current that the switch conducts at the beginning of the switch-on period. Additionally, R is the on-resistance of the switch, ton is the conduction period, and fsw is the switching frequency. The product of ton·fSW is called duty cycle. For a given output power and a given switching frequency, a lower input voltage can result in larger duty cycle thus higher conduction loss.
Moreover, for the flyback power conversion system 100, the power P delivered to the output is, for example,P=½fSW·L·(IPK2−IPK2(0)  (6)
and
                                          I            PK                    -                                    I              PK                        ⁡                          (              0              )                                      =                                            V              IN                        L                    ·                      t            on                                              (        7        )            
where L is the inductance of the primary winding, and VIN is an input voltage for the primary winding. For example, the input voltage VIN is the rectified line voltage. For a given output power P, increasing the switching frequency fsw can result in decreasing magnitude for (IPK2−IPK2 (0)), thus lower conduction loss according to Equation 5.
Therefore, it is highly desirable to improve techniques related to conversion efficiency of a power conversion system.